Evaporation heat exchanger, especially for a spacecraft

ABSTRACT

An evaporation heat exchanger having at least one active circulating circuit for a coolant liquid to be cooled, is constructed for removing heat in a spacecraft under a gravity-free operating condition and under different acceleration conditions. For this purpose, a housing encloses an evaporating space provided with at least one inlet valve for controlling the admission of a medium to be evaporated. The produced vapor or steam is discharged through a respective port. A plurality of cooling tubes interconnected in a meandering shape form a number of tube panels in which the individual tubes are arranged in a zig-zagging pattern to form respective flow passages for the medium to be evaporated. The individual tubes extend in parallel to each other and perpendicularly to the flow direction of the medium to be evaporated, while the meandering configuration is forming the zig-zag pattern. This arrangement of the cooling tubes assures a high efficiency in the heat transfer and also in the complete conversion of the liquid to be evaporated into vapor or steam. Due to the zig-zag pattern, the medium to be evaporated, or droplets thereof, are repeatedly intercepted and deposited by the exchanger surfaces of the tubes so that the exchanger functions as a drip-catcher evaporator.

FIELD OF THE INVENTION

The invention relates to an evaporation heat exchanger, especially for cooling the cooling liquid in circulating circuits in spacecraft under gravity free conditions and under different acceleration conditions.

BACKGROUND INFORMATION

Ascending or descending spacecraft passing through the earth's atmosphere, or spacecraft orbiting in an orbit around the earth, are exposed to extreme thermal stress conditions. Therefore, it is necessary to provide a safe and reliable heat discharge.

German Patent Publication (DE-PS) 3,718,873 describes an evaporating heat exchanger suitable for the above stated purpose of cooling a spacecraft. In the known heat exchanger the medium to be cooled circulates in an active liquid circulating circuit for heat exchanges. For this purpose, the medium to be cooled is brought into a heat transfer contact with a medium to be evaporated. The medium t be evaporated is carried in a supply container in the spacecraft. The generated vapor is blown off from the spacecraft into its environment.

It is desirable to evaporate the cooling medium as much as possible to use it with an optimal efficiency. For this purpose it is further desirable to achieve a sufficiently high heat transfer between the cooling medium to be evaporated and the flow channels which carry the liquid to be cooled. To perform these two functions efficiently, it is necessary that the generated vapor is separated from the medium portion which is still liquid to make sure that liquid portions of the evaporating medium are not discharged from the spacecraft in the form of large liquid drops which have not participated in the evaporation, and thus in the cooling process.

In the above mentioned German Patent Publication (DE-PS) 3,718,873 it is the aim to deposit non-evaporated liquid remainders with the aid of mass inertia forces on the walls of the cooling liquid channels. For this purpose, the cooling liquid channels are arranged in a folded pattern as viewed in the flow direction. The so deposited liquid remainders are then transferred into the gas phase on the surface of these channels. However, tests performed with the known heat exchanger have shown that the utilization of the cooling medium to be evaporated is not at all optimal and hence not efficient for the intended purpose.

OBJECTS OF THE INVENTION

In view of the foregoing, it is the aim of the invention to achieve the following objects singly or in combination:

to construct an evaporating heat exchanger of the type described above in such a way that the medium to be evaporated is practically completely transferred into the vapor phase to assure its efficient utilization for an optimal heat exchange;

to make sure that only vapor is discharged from the spacecraft and that larger liquid drops are retained for an efficient use of the evaporating medium;

to arrange flow channel forming tubes carrying the medium to be cooled in such a way that an optimal utilization of the available space is assured; and

to shape and arrange the cooling liquid carrying tubes in such a way as to optimally increase the heat exchanging contact between these tubes and the medium to be evaporated.

SUMMARY OF THE INVENTION

The heat exchanger according to the invention is characterized in that the cooling liquid channels comprise bundles of tubes extending perpendicularly to the main flow direction of the medium to be evaporated and parallel to one another, whereby pairs of these tubes which follow one another in the main flow direction, are located in alternatingly displaced positions relative to one another and relative to a plane extending in the main flow direction. Thus, these tube bundles form tube panels which zig-zag relative to said main flow direction.

The present evaporating heat exchanger functions in the manner of a drop separator due to the arrangement of the liquid channels in the form of tube bundles extending in a direction across or perpendicularly to the main flow direction and because the tubes in each individual plane are arranged in alternatingly displaced positions relative to one another. Several tube groups form several tube panels in a plurality of planes. These features make sure that the entire surface area available for causing the evaporation is used as completely as possible so that these liquid channel surfaces achieve a practically complete evaporation of the entire medium fed into the evaporator.

Additionally, the invention has the following advantages. A high efficiency is achieved in a small volume, which in turn is achieved due to the high packing density of the cooling tubes. The arrangement of the tubes is simple and hence easily manufactured. Further, the maintenance and periodic check-up work is facilitated by the easy access to all components of the heat exchanger.

The special arrangement of the individual cooling tubes serves for a further optimizing of the heat transfer characteristics, while simultaneously assuring a low pressure loss of the cooling circulating circuit as well as of the medium to be evaporated on its way from the inlet valve to the exit or discharge opening. These features minimize the required pumping power and the evaporating temperature can be rather low at the prevailing low pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 is a side view into an open side of the present heat exchanger;

FIG. 2 is a view in the direction of the arrows II in FIG. 1, showing one panel of cooling tubes arranged according to the invention in a meandering zig-zag pattern;

FIG. 3 is a view approximately in the direction toward the plane indicated by the arrows III in FIG. 1;

FIG. 4 is a side view of a section of an individual cooling tube;

FIG. 4a is a sectional view along section plane A--A in FIG. 4;

FIG. 4b is a sectional view along section plane B--B in FIG. 4;

FIG. 4c is a sectional view along section plane C--C in FIG. 4; and

FIG. 5 is a view similar to that of FIG. 2, but showing, on a somewhat enlarged scale, three panels nesting one within the other to provide an efficient utilization of space within the heat exchanger housing.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE OF THE INVENTION

In the FIGS., the same components are provided with the same reference numbers.

FIG. 1 shows a schematic longitudinal section through a heat exchanger 1 comprising a housing 2 merely shown by dash-dotted lines to reveal the internal arrangement of the cooling tubes 4 to 13 forming an active coolant circulating circuit 3 containing, for example water or a cooling liquid like refrigerant R114 known under the Tradenames Freon or frigen 114 flowing in a coolant circulating circuit of a spacecraft. The liquid to be cooled when it passes through the tubes 4 to 13 may also, for example, be the hydraulic oil of the hydraulic system of a spacecraft. The individual tubes 4 to 13 form a bundle or panel 14 to be described in more detail below.

The housing 2 with its cover 2c encloses an inlet space 15 communicating through inlet valves 16 and 17 with a supply of a cooling medium to be evaporated for cooling the coolant passing through the tube bundle or panel 14. The supply container for the cooling medium to be evaporated is not shown. However, the evaporating liquid may be water and/or ammonia or the like. The opposite end of the housing 2 is formed as a vapor discharge port 18. The flow direction of the cooling medium is indicated by the arrow S.

The active cooling circuit 3 has an inlet port 19 and an outlet port 20 interconnected through a plurality of tube bundles or panels 14. These bundles or panels 14 are arranged relative to the flow direction S as will be explained in more detail below with reference to FIGS. 2 and 5.

As shown in FIG. 2, the bundle or panel 14 of tubes 4 to 13 have long leg straight tube sections and short leg tube sections interconnecting the long tube legs. The short tube legs are, for example, semi-circular tube sections to form the meandering pattern shown in FIG. 1. Additionally, according to the invention, each short tube leg is angularly displaced by an angle α as shown in FIG. 2. The angle α extends between a plane 14' and a direction defined by the respective short tube leg. The plane 14' in turn extends in the direction of the main flow S and the long tube legs extend substantially perpendicularly to said main flow. In FIG. 1, the plane 14' may, for example, be the plane defined by the drawing sheet. As a result, the short legs of the tubes 4 to 13 in each panel 14 zig-zag through the housing 2 and, as shown in FIG. 5, neighboring panels 14 form zig-zagging flow path 14a, whereby the zig-zagging increases the flow path length for the medium to be evaporated. Only three panels are shown in FIG. 5. However, the number of panels may be substantially larger if desired. All the individual panels are interconnected at the inlet end to a manifold pipe 21 and at the outlet end to a manifold pipe 22. The manifold 21 is connected to the inlet port 19 while the manifold 22 is connected to the outlet port 20. The medium to be cooled flows into the inlet port 19 and out of the outlet port 20, which form part of the closed circulating circuit 3. As further shown in FIG. 1, the housing 2 is supported by mounting pads 2a and 2b.

Referring further to FIGS. 2 and 5, the angle α should be about 15°, whereby the folding angle β between two neighboring pairs of tubes, becomes about 150°. The resulting zig-zagging permits the nesting of the zig-zag of one panel within the zig-zag of the neighboring panels so that directly neighboring short zig-zag tube legs of all neighboring panels extend respectively in parallel to each other without any portion of any tube panel reaching into any free space of any other tube panel and with the required spacing between neighboring tube panels 14, to form the zig-zagging flow paths 14a as best seen in FIG. 5.

The view of FIG. 3 shows that, for example, twenty-five panels 14 are arranged in parallel to each other in the housing 2. As seen in FIG. 1, each panel 14 comprises ten tubes 4 to 13 so that a total of two-hundred and fifty tubes are easily accommodated in a relatively small volume. Yet, due to the zig-zagging arrangement, the present evaporator is capable of operating as a so-called TRAP (i.e. droplet trap evaporator).

Referring to FIG. 4, the individual tubes 4 to 13 are provided with indentations 23, 24, and 25. FIG. 4a shows the indentation 23 at the sectional plane A--A. FIG. 4b shows the oval, somewhat flattened cross-section of the tubes along section plane B--B.

FIG. 4c shows the indentation 24 along section lines C--C in FIG. 4. The on-center spacings D between neighboring indentations 23, 24, are preferably equal along the length of each tube so that the indentations 23, 25 on one side of the tube are staggered or phase shifted relative to the indentations 24 on the opposite side of the tube. Additionally, neighboring indentations are arranged on opposite sides of the tube as is apparent from FIB. 4. Prior to making the indentations 23, 24, 25, the flat or short diameter E of the tubes as shown in FIG. 4b is, for example, 2.51 mm. the indentations are then formed into the flat sides of the tubes. The indentations cause eddy currents which in turn contribute to a good heat transfer characteristic. The spacing D between neighboring indentations is, for example, 4.25 mm. Each tube 4 to 13, outside the semi-circular interconnecting tube sections, has, for example, a length of 95 mm. As mentioned, with ten tubes in each of twenty-five tube panels, a total of two-hundred and fifty tubes is easily accommodated in a relatively small volume which is advantageous for achieving a highly efficient heat transfer and hence cooling.

The inlet valves 16 and 17 permit controlling the volume flow of the medium to be evaporated, for example, water. These valves 16 and 17 may in turn be controllable by an electromagnet energized in a pulsating manner for opening these valves for short time durations as required under particular operating conditions.

The medium to be evaporated is introduced into the space 15 in the housing 2 under a supply pressure which is above the boiling pressure in the evaporation space 15. Due to the low pressure prevailing in the space 15, a small proportion of the medium to be evaporated does evaporate already in the space 15 in an adiabatic manner, whereby the liquid of the medium to be evaporated is cooled down (so called flash evaporation). Where water is the medium to be evaporated, about 5 to 8% of the water mass is cooled in this adiabatic manner. This initial evaporation in the space 15 produces gas which flows to the exhaust and drags the remaining liquid droplets, thereby generating an accelerated two-phase flow by the gas expansion. If the cooling medium is water having a temperature of 25° C., the two-phase flow comprises about 99.95% water vapor by volume and about 0.05% by volume of liquid water if the pressure in the evaporating space 15 is within the range of about 5 to 10 mbar. The accelerated two-phase flow now contacts the tubes 4 to 13 forming the tube bundles or panels 14 of the cooling circulating circuit 3. Incidentally, several circuits 3 can be formed to provide a plurality of separate flow circuits for separate cooling liquids to be cooled. The cooling liquid to be cooled in the circuit or circuits 3 transports in a known manner heat from heat producers to the cooling channels 14a formed by the panels 14 providing the actual heat exchanger surfaces at which the heat is taken up by the medium to be evaporated so that the latter is actually evaporated and the resulting vapor or steam exits through the discharge outlet 18 of a spacecraft or the like.

In operation, as the liquid droplets still present in the gas phase are entrained to travel along with the gas phase, these droplets tend to deviate from the main gas flow direction S due to the fact that the droplets have a larger mass inertia than the gas phase, whereby the droplets impinge on the outer tube surfaces of the cooling channels 14a formed by two neighboring panels 14. On these surfaces the liquid droplets form a liquid film which partially evaporates. Due to the staggering caused by the zig-zagging shape of the panels 14, the arrangement functions in the manner of a heated droplet trap evaporator, whereby the tubes 4 to 13 of the panels functions as deflector surfaces. At the folding points, namely where the tubes are located, the portions of the liquid film not yet evaporated, form new droplets which are entrained by the two-phase flow and are again caused to impinge on the outer surfaces of the cooling liquid channels 14a. Thus, the two-phase flow is repeatedly divided, whereby, according to the invention, the zig-zagging flow direction causes an especially intensive contacting of the medium to be evaporated with the surfaces to be cooled. As a result, the apparatus according to the invention achieves a complete evaporation with an excellent efficiency in the heat transfer.

Although the invention has been described with reference to specific example embodiments, it will be appreciated that it is intended to cover all modifications and equivalents within the scope of the appended claims. 

What I claim is:
 1. A heat exchanger for cooling a coolant in a cooling circuit in a spacecraft under gravity-free operating conditions and under different acceleration conditions, comprising a housing enclosing an evaporation space, a plurality of tube panels in said evaporation space, inlet valve means for introducing an evaporating medium into said housing and into said evaporation space, vapor outlet means for discharging vapor from said housing, a plurality of cooling tubes forming said tube panels in said housing, short tube legs interconnecting said cooling tubes to form meandering tube loops forming part of said cooling circuit, said cooling tubes having long legs extending in parallel to one another and substantially perpendicularly to a main flow direction of said evaporating medium from said inlet valve means to said vapor outlet means, said short tube legs of said cooling tubes being arranged so that two neighboring tubes in a bundle of tubes forming a tube panel of said plurality of tube panels, are alternatingly displaced laterally relative to a plane extending in said main flow direction, to form each tube panel with a zig-zag pattern, and wherein all tube panels are arranged in parallel to each other so that zig-zagging flow paths (14a) for said evaporating medium are formed between neighboring tube panels without any portion of any tube panel reaching into any free space in any other tube panel, whereby contact between said evaporating medium and cooling surfaces of said tubes is effectively increased while still permitting a substantially free flow of said evaporating medium through said zig-zagging flow paths for an efficient evaporation.
 2. The heat exchanger according to claim 1, wherein said tubes have a flattened oval cross-section.
 3. The heat exchanger according to claim 2, wherein each flattened tube has additional indentations for an improved heat transfer.
 4. The heat exchanger according to claim 3, wherein said indentations are located alternately on opposite sides of the respective tube, and wherein said indentations are uniformly spaced from one another along opposite sides of said tubes.
 5. The heat exchanger according to claim 4, wherein said indentations along one side of said tubes are phase shifted in the longitudinal tube direction relative to indentations on the opposite side of said tubes.
 6. The heat exchanger according to claim 1, wherein said bundles of tubes forming said cooling circuit are arranged to be contacted by said evaporating medium sprayed as a liquid to be evaporated in said housing, said liquid to be evaporated being enriched with a wetting agent for an improved wetting, and wherein the spraying of the liquid to be evaporated takes place through an adiabatic expansion evaporation of the evaporating liquid which is super-cooled in a supply conduit leading to said inlet valve means, said evaporating liquid being super-heated relative to a pressure prevailing in an inlet space of said housing.
 7. The heat exchanger according to claim 1, wherein at least certain of said cooling tubes contact walls of said housing for avoiding freeze-up.
 8. The heat exchanger according to claim 1, wherein said evaporating medium is water.
 9. The heat exchanger according to claim 1, wherein said evaporating medium is ammonia.
 10. The heat exchanger according to claim 1, wherein said evaporating medium is water and ammonia used sequentially. 